Abstract: Abstract: With the increase in labor costs and the development of peanut large-scale production in China, the demand for high-efficient peanut combine harvesters has become increasingly urgent. 4HLB-4 type semi-feeding four-row peanut combine harvester can harvest 2 ridges (4 rows) at one time, whose productive efficiency is more than twice that of the existing semi-feeding two-row peanut combine harvester. At present, simple mechanical depth-control devices such as depth-control plates and depth-control wheels have been widely used on the semi-feeding peanut combine harvester, but in practice, it is found that simple mechanical depth-control devices are difficult to meet the requirements of precision and high efficiency. In the application of semi-feeding four-row peanut combine harvester, there are many problems such as slow response and poor accuracy, which easily result in high peanut leakage rate and breakage rate. In order to overcome the above problems, in this paper, a set of automatic depth control device for 4HLB-4 type semi-feeding four-row peanut combine harvester was designed by adopting electronic sensor technology, hydraulic driving technology and microprocessor control technology comprehensively. The device was composed of a depth-control profiling mechanism, an excavation depth adjustment mechanism, a hydraulic execution system, a microprocessor control system and a control software. The working principle of the device is as follows: The depth-control profiling mechanism senses the change of the ridge top undulation, converts the fluctuating changing amount into the angle information, and then transmits it to the angle encoder. The angle encoder converts the angle information into a digital signal and transmits it to the controller. Based on the compiled program, the controller analyzes the angle signal and then outputs the solenoid valve control signal to open the hydraulic cylinder to drive oil circuit. The hydraulic cylinder drives the excavation depth adjustment mechanism to adjust the height of the excavating shovel. At the same time, an articulated linear displacement sensor installed in parallel with the hydraulic cylinder of the harvesting platform is used to measure the displacement of the hydraulic cylinder and feed it back to the controller. According to the processing result of the displacement, the controller adjusts the output control signal to ensure that the depth of excavation is stable so that the entire automatic depth-control system forms a closed loop feedback system. In this system, the controller is a single-chip microprocessor IAP15W4K58S4. The control software program adopts the fixed dual-threshold dead-zone PID (proportion, integral, derivative) control algorithm, and is compiled by C language in Keil uVision4. It increases system noise immunity, avoids frequent shocks of control action, and improves control stability. The field harvesting test showed that the automatic depth-control system was stable and reliable. After 4HLB-4 type semi-feeding four-row peanut combine harvester adopted the system, the average leakage rate was 1.08%, the average breakage rate was 0.94%, and the average excavation depth was 123 mm. Compared with the artificial depth-control harvesting test results, it was found that the average leakage rate decreased by 2.13 percentage points, the average breakage rate decreased by 1.4 percentage points, the average excavation depth deviation decreased by 11 mm, and the excavation depth deviation was more stable. This study can provide some reference for the development of automatic depth-control systems for other under-soil fruit harvesting machines.